This invention relates generally to a channeling method and apparatus for introducing additional service in the TACAN frequency band and more particularly for introducing precision DME for a microwave landing system (MLS), wherein said channeling scheme may be operated independently of the present TACAN system or in conjunction with it.
The present TACAN/VORTAC/DME system (hereinafter referred to as TACAN) utilizes an airborne transmitter operating in the frequency range of 1025-1150 MHz. The conventional airborne transmitter transmits a pair of Gaussian shaped interrogation pulses, the pulses of each pair spaced 12 microseconds apart for X mode operation and 36 microseconds apart for Y mode operation. Conventional transponders monitor the environment and upon receipt of on channel interrogation pulses generate trigger pulses which, in turn, initiate reply pulses. If X mode interrogations are received, the reply signal consists of a pair of Gaussian pulses spaced 12 microseconds apart. These reply pulses are transmitted in either the 962-1024 MHz or 1151-1213 MHz frequency range. If Y mode interrogations are received, the reply signal consists of a pair of Gaussian pulses spaced 30 microseconds apart. Y mode reply signals are transmitted in the 1025-1150 MHz frequency range. In every case, the difference between the transmit frequency and receive frequency is .+-. 63 MHz. This is true for the airborne equipment and the ground equipment, and represents a very convenient arrangement since the same RF source used to drive the transmitter can also be used as the receiver local oscillator provided that the receiver's first IF stage is 63 MHz.
The conventional channeling arrangement described above also permits ranging between airborne sets, each of which transmit Gaussian pulse pairs and receive single Gaussian pulse replies. This is commonly referred to as the Air/Air mode. All transmission and reception in this mode occurs in the 1025-1150 MHz frequency range, and the difference in transmit and receive frequencies is also 63 MHz. Complete details can be found in MIL-STD 291.
In conventional L-Band DME systems, standardized by the International Civil Aviation Organization (ICAO), interrogations and replies comprise pairs of Gaussian pulses each having a duration of 3.5 microseconds and having rise times in the order of 2.5 microseconds. The resulting accuracy, limited by this rise time, is less than what may be desirable for applications associated with the microwave landing system (MLS) which has been suggested by a special committee of the Radio Technology Commission for Aeronautics. Care must be taken, however, that any new high accuracy DME system operating an L-Band will not interfere with the currently operating and conventional NAV/ILS L-Band system and further, will not be interferred with by the currently operating system.